1. Technical Field
Example embodiments of the inventive concept relate to semiconductor memory devices. More particularly, example embodiments of the inventive concept relate to non-volatile memory devices having multi level cells.
2. Discussion of Related Art
A semiconductor memory device may be classified into tow types (i.e., a volatile memory device and a non-volatile memory device) according to whether data can be retained when power is not supplied. Recently, a NAND flash memory device having multi level cells (Hereinafter, a MLC NAND flash memory device) is widely used as the non-volatile memory device because the NAND flash memory device can be manufactured smaller in size while having higher capacity.
The NAND flash memory device performs a write operation and a read operation by a page unit. A read margin of a threshold voltage state distribution is not sufficiently secured when the MLC NAND flash memory performs the read operation of page data, on that read errors occur. Further, the read errors may gradually increase due to the threshold voltage state distribution becomes broader with the passage of time.
For this reason, the MLC NAND flash memory device has employed a technique for adjusting a read voltage (or a readout voltage) in order to reduce the read errors and to improve the reliability of the page data that are read out by the read voltage. However, when the MLC flash memory device is employed the technique for adjusting the read voltage, the operations for adjusting the read voltage have to be separately performed in addition to the basic operations (e.g., the read operation, the write operations, an erase operation, etc). Therefore, the typical MLC NAND flash memory device is difficult to operate at a high speed.